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doi:10.2204/iodp.proc.336.104.2012 Site U13821Expedition 336 Scientists2Site summaryIntegrated Ocean Drilling Program (IODP) Expedition 336 Hole U1382A was drilled 50 m west of Deep Sea Drilling Project (DSDP) Hole 395A at 22°45.353′N, 46°04.891′W, in 4483 m water depth. The primary objective for Hole U1382A was to install a subseafloor borehole observatory (CORK) to perform long-term coupled microbiological, biogeochemical, and hydrological experiments in uppermost basaltic crust in this area of very low conductive heat flow. Coring and downhole logging of basement were also conducted. After the reentry cone with 53 m of 16 inch casing was jetted in, the hole was deepened by drilling with a 14¾ inch tricone bit to 110 meters below seafloor (mbsf) without coring. Basement was encountered at 90 mbsf, and 3 m of basement was penetrated in 30 min. The interval from 93 to 99 mbsf was drilled very quickly and is inferred to be sediment; however, the underlying formation to 110 mbsf was drilled slowly (2–3 m/h) without significant torque. Casing (10¾ inch) was installed and successfully cemented to 102 mbsf. Rotary core barrel (RCB) coring recovered basement from 110 to 210 mbsf (Cores 336-U1382A-2R through 12R). In total, 32 m of core was retrieved, with recovery rates ranging from 15% to 63%. This succession resembles the lithostratigraphy encountered in DSDP Holes 395 and 395A and provided excellent sampling material for various microbiological and petrologic studies. The shipboard petrologists divided the core into eight lithologic units, comprising numerous subunits. Major unit boundaries are defined by contacts between massive and pillowed flows and interlayered sedimentary units. Each major lava flow unit consists of several cooling units, which are recognized by glassy or variolitic margins or marked changes in grain size. Results from thin section studies reveal a large range of grain sizes (glassy to medium grained) and diverse textures (aphanitic to subophitic or intersertal). Basalts are either aphyric or plagioclase-olivine-phyric and have <3% vesicles. Phenocryst contents range up to 25%, with plagioclase being more abundant than olivine. All of the volcanic rocks recovered from Hole U1382A are affected only by low-temperature alteration by seawater, manifesting as replacement of groundmass and phenocrysts, vesicle filling, glassy margin replacement, and vein formation with adjacent brown alteration halos. Chilled margins often show advanced palagonitization, which develops as blotchy alteration texture following the primary variolitic texture of the mesostasis. The extent of alteration ranges up to 20%, with clay (smectite and celadonite) being the most abundant secondary phase, followed by Fe oxyhydroxides and minor zeolites and carbonates. The recovered section has between 13 and 20 veins/m, with vein thickness being usually <0.2 mm. A sedimentary unit in Cores 336-U1382A-8R and 9R features a variety of clasts, including plutonic and mantle rocks. The peridotites are weakly serpentinized harzburgites and lherzolites with a protogranular texture. The intensity of deformation of the gabbroic lithologies ranges from undeformed to mylonitic. Minor cataclastic deformation of the peridotites has led to the development of carbonate-filled vein networks, along which the rocks have been subjected to oxidative alteration, resulting in the breakdown of olivine to clay, oxide, and carbonate. Physical properties measurements reveal typical P-wave velocities for these lithologies and a correlation between sonic velocity and porosity of the basalt. Elevated potassium and uranium concentrations in the oxidatively altered part of the core were revealed by natural gamma radiation (NGR) core scanning. Thermal conductivity also reflected the typical values associated with basalt and peridotite and showed small variations with depth. Whole-rock geochemistry analyses reveal systematic differences in compositions between aphyric and porphyritic basalt, which are due to plagioclase accumulation in the porphyritic basalt. The aphyric basalts show a liquid line of descent that is controlled by the fractional crystallization of olivine. As the extent of alteration increases, weight loss on ignition (LOI) values and potassium concentrations also increase. Immobile trace element ratios (Zr/Y and Ti/Zr) indicate that parental magma compositions for the basalts above and below the sedimentary unit are different from each other. Petrographically and geochemically, the basalts correspond to the uppermost lithologic units identified in Hole 395A. Likewise, a sedimentary unit with varied plutonic and mantle rocks was also observed in Hole 395A. A primary objective of basement coring was to obtain samples for microbiological analysis. We collected 46 hard rock and 2 sediment whole-round samples for these studies (11% of core recovered). Samples were preserved for ship-based (deep ultraviolet [UV] fluorescence scanning, culturing and enrichment, and fluorescent microsphere analysis) and shore-based (DNA and RNA analysis, fluorescence in situ hybridization [FISH], cell counting, and isotopic analysis) studies. Generally, 1–3 microbiology hard rock samples were collected from every core section. Hard rock samples span a range of lithologic units, alteration states, and presence of chilled margins, and some contain veins or fractures. Additionally, a few recovered plastic bags that held the fluorescent microsphere solutions in the core catcher were collected as a contamination check via DNA analysis. An open-hole section of 105.61 m was logged over a period of ~19.5 h with two tool strings (the adapted microbiology combination I [AMC I] and the Formation MicroScanner [FMS]–Hostile Environment Natural Gamma Ray Sonde [HNGS]). Downhole log measurements include natural total and spectral gamma radiation, temperature, density, electrical resistivity, electrical images, and deep UV–induced fluorescence (acquired with the new Deep Exploration Biosphere Investigative tool [DEBI-t]). The borehole remained in good condition throughout logging, and no obvious tight spots were encountered in open hole. Integration of core and log measurements and observations showed excellent correspondence between potassium concentrations provided by shipboard NGR, the spectral gamma ray logging tool, and whole-rock geochemical analyses. FMS data were combined with images of the external surfaces of whole-round cores. Prominent veins with alteration halos in cores from the massive flows were matched with fractures in the FMS images. Also, logging results constrain the depth of the peridotite interval from 165 to 167 mbsf (based on density and low K/U ratios). Downhole hydrologic (packer) tests failed because ship heave up to 3 m prevented the packer from sealing in the casing for more than 10 min. A CORK to monitor and sample a single interval in uppermost basement was successfully installed in Hole U1382A. The 210 m deep hole was sealed with a 189 m long CORK with nine external umbilicals and a retrievable internal instrument string. The umbilicals include one for pressure monitoring, two for microbiological sampling, and six for fluid sampling. The retrievable internal instrument string comprises several osmotic pump-driven samplers for basement fluids and microorganisms as well as enrichment experiments, an oxygen probe, and a thermistor with data recorder. The samplers and probes extend from 146 to 174 mbsf and are kept in position by a 150 lb sinker bar at 177 m. A pressure gauge and a fast-pumping OsmoSampler are situated in the wellhead and monitor/sample fluids from 162 mbsf. |
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